Modern gas turbine engines, particularly those used in aircraft, operate at high rotational speeds and high temperatures for increased performance and efficiency. The turbine of a modern gas turbine engine is typically of an axial flow design and includes a plurality of axial flow stages. Each axial flow stage comprises a plurality of blades mounted radially at the periphery of a disk which is secured to a shaft. A plurality of duct segments surrounds the stages to limit the leakage of gas flow around the tips of the blades. These duct segments are located on the inner surface of a static housing or casing. The incorporation of the duct segments improves thermal efficiency because more work may be extracted from gas flowing through the stages as opposed to leaking around the blade tips.
Although the duct segments limit the leakage of gas flow around the blade tips, they do not completely eliminate the leakage. It has been found that even minor amounts of gas flow around the blade tips detrimentally affect turbine efficiency. Thus, gas turbine engine designers go to great lengths to devise effective sealing structures. These structures generally include a coated duct segment in combination with a blade tip coating which renders the tips resistant to wear. In operation, the tips provide sealing by cutting into the coating on the duct segment. Thereby preventing damage to blades and resulting in minimum possible tip clearances and air leakage.
Unfortunately current duct segment coatings, which are typically ceramic, suffer from excessive material loss as a result of erosion or spalling. In general, erosion is the wearing away of coating material due to factors such as abrasion and corrosion. Erosion often results from particle impingement during engine operation. Spalling or spallation is typically caused by delamination cracking at the ceramic-metal interface resulting from thermal stress and the aggressive thermal environment. Spalling is essentially piecemeal coating loss consisting of many small coherent volumes of coating material.
The coating losses due to erosion and spallation result in large part to microcracks present in the segment ceramic coating. Microcracks formed parallel to the substrate surface, or horizontal microcracks, causes the coating to spall off when subjected to the above mentioned operating conditions and environment. In contrast, vertically oriented microcracks bolster the coating's strain tolerance which prolongs the coating's service life. The mechanism of microcrack formation in segmented ceramic coatings is thermally induced stress. Thermal gradients are induced into the coating in a cyclic manner during coating deposition. These gradients are controlled to allow coating to be applied to a surface with no open cracks, then as each thin layer is built up and subsequently cooled, surface shrinkage produces stress levels required for cracks to propagate to the surface. Reheating of the surface then closes the cracks prior to the next thin layer of coating being applied. The relative tendency of cracks to propagate through their thickness or parallel to the substrate is dependent upon the thickness of the layers that are applied before crack propagation is induced.
Ceramic coating loss increases blade tip clearance and thus is detrimental to turbine efficiency, as well as detrimental to the blades themselves. For example, the blades may become damaged due to the increased temperature at which the engine must then operate to make up for lost thrust. Such performance losses may be prevented by improving the quality of the segmented ceramic coating.
Presently, U.S. Pat. No. 6,102,656 ('656 patent) discloses one such method of applying a segmented ceramic coating in an effort to improve the ceramic coating. Applying ceramic coatings upon substrates is an automated process whereby the substrate is placed in a fixture that rotates about an axis or moves in a linear direction along a conveyor for example.
As described in the '656 patent, a substrate 10 may move in a direction indicated by an arrow 12 in such an automated process (see FIG. 1). A plasma torch apparatus 14 moves in a direction opposite substrate 10 as indicated by an arrow 26 and emits a plasma plume 16. Plasma plume 16 is defined by a pair of solid lines that is directed towards a surface 18 of substrate 10. Plasma torch apparatus 14 includes a ceramic (or powdered) material feeder (not shown) that emits a quantity of ceramic material 20 in a direction indicated by an arrow 28 into plasma plume 16. Ceramic material 20 becomes entrained within plasma plume 16 and is carried towards surface 18. As illustrated, plasma plume 16 comprises a much broader spray pattern than ceramic material 20 such that a deposition area 22 forms within a heated area 24 on surface 18.
The '656 patent relies upon a high power level and gas flow utilized in conjunction with a slow relative motion of the plasma torch to the parts (substrate 10) to produce the surface heating by the plasma and air cooling necessary to achieve vertical microcracking. These conditions represent a compromise between equipment capability, efficiency and microstructural characteristics of the ceramic coating.
The current process may not always exert adequate active control of the thermal gradients and thermal cycling that occurs in the spray process. The balance between vertical crack formation and horizontal crack formation is very difficult to control and occurs very randomly. As depicted in FIG. 1, the heating zone 24 created by the plasma torch is larger than the deposition area 22, and extends further over the substrate in the direction where deposition has just taken place than where deposition is about to take place, i.e., the heated area 24 extends further to the left of deposition area 22 than to the right of deposition area 22 in FIG. 1. Due to this relationship between surface heating and deposition location, only a moderate driving force is present to propagate through thickness cracks. As a result, shrinkage occurs in more than one direction and thermal cycling causes cracks to form horizontally within the plane of the coating as well as vertically through the coating. The horizontal cracks that are parallel to the substrate do not improve the coating's strain tolerance and durability; these cracks actually cause the coating to spall off.
Consequently, there exists a need for an improved method for controlling the crack formation in segmented ceramic coatings, thereby improving process repeatability and consistency of coating performance, as well as for facilitating an independent control of cracking and porosity.